. Applicant's Abstract: Accumulating evidence demonstrates a significant role for TNF-alpha in the hepatocellular disruption associated with xenobiotic exposure. Preliminary studies support that TNF-alpha may play a role in dimethylnitrosamine (DMN) toxicity as well. Experiments described in this proposal will qualify the contribution of TNF-alpha receptor (TNFR) signals to pathophysiologic processes and delineate the mechanisms by which TNF-alpha contributes to DMN-induced hepatoxicity. Specifically, the investigators will utilize genetic knockout mice deficient for one or both TNFR, TNFR55 and TNFR75, to determine how hepatocellular damage is controlled by specific TNFR signals. Histopathological and biochemical measures of liver damage during DMN exposure in vivo will permit assignation of specific hepatocellular events to TNFR signals. For TNFR knockout mice exposed to DMN, molecular analyses will quantitate changes in hepatic transcript levels for TNFR55, TNFR75, and genes which control reactive oxygen and nitrogen species formation. Moreover, the investigators will use the powerful differential display reverse transcription-PCR (DDRT-PCR) technique to identify novel TNF-a regulated genes contributing to DMN hepatoxicity. Thus, the impact of specific TNFR signals on hepatic gene expression during DMN exposure can provide for linkage between liver injury and genetic responses attributable to TNF-alpha. Finally, experiments will determine whether TNF-alpha is a central mediator for tissue destruction induced by a variety of hepatotoxicants. Liver histopathology will be evaluated in TNFR deficient mice exposed to CCl4 acetaminophen, cadmium, and chlorinated hydrocarbons to determine whether neutralization of specific TNF-a signals impacts tissue damage and gene expression in a manner consistent with the DMN model. These data will delineate hepatotoxicant mechanisms regulated via a central TNF-a pathway. That TNF-alpha contributes to hepatotoxicant-induced liver damage will be evaluated based on the hypotheses that (1) Removal of TNFR signals reduces DMN hepatotoxicity; (2) TNF-alpha regulates gene expression reflective of hepatocellular damage; and (3) TNF-alpha is a central pathway contributing to xenobiotic-induced hepatotoxicity.